The Agnew-Wiluna greenstone belt of Western Australia is the largest komatiite-hosted nickel sulfide belt in the world and contains two world-class Ni-Cu-(PGE) deposits and a host of smaller deposits. This study focuses on the broader scale geology of this greenstone belt in order to understand the key controls on the genesis of the komatiite-hosted Ni-Cu-(PGE) deposits, with specific focus on camp to district controls. We apply multiple sulfur isotopes to this geologic framework and conclude not only that the addition of crustal sulfur is a prerequisite for ore genesis in komatiite systems, but above all that the sulfur required to generate world-class deposits is most likely derived from barren volcanic massive sulfide lenses, which are spatially and genetically associated with felsic volcanic and volcaniclastic sequences that were emplaced coevally with large komatiitic sills and channelized lava flows. Multiple sulfur isotope data can be utilized in exploration at the deposit to district scales. At the deposit scale, the spatial pattern of mass-independent S isotope values (Δ 33 S) provides crucial insight into the identification of proximal high-grade and high-tenor ores in mineralized systems. In fact, sulfur data reflect the assimilation process that occurred upon komatiite emplacement, whereby hot turbulent magma thermomechanically eroded and assimilated exhalative sulfides spatially located close to vent with negative to near zero Δ 33 S values, whereas less turbulent flows interacted with distal sulfidic shales having Δ 33 S values above 0 per mil. Accordingly, the spatial variation of multiple sulfur isotope values in magmatic sulfides and associated host rocks may be utilized as a vector towards high-grade ores of poorly known systems. At the district scale, rather than ascertaining what controls the distribution of komatiite-hosted Ni-Cu-(PGE) deposits, the appropriate question to ask is what controls the distribution of country rock sulfides, considering that exhalative sulfides may be crucial to ore genesis in komatiite systems. We propose that felsic lava domes unambiguously mark their vents and can be directly mapped or inferred from gravity data. This work provides the first step in identifying district-scale control on komatiite-hosted Ni-Cu-(PGE) deposits. This is the scale that has high impact on exploration for new komatiite-hosted nickel sulfide belts globally.
Abstract The Paleoproterozoic Mârmorilik Formation in the Karrat basin of West Greenland hosts the Black Angel Zn–Pb deposit. Chlorine-rich scapolite, zones with vuggy porosity and quartz nodules in the ore-bearing marble are herein interpreted to represent metamorphosed, vanished, and replaced evaporites, respectively. Mineralization is closely associated with anhydrite with δ 34 S values (5.2–12.6‰) broadly comparable to published values for Paleoproterozoic seawater sulfate. Considering the fundamental attributes of the mineralization and host sequence, a Mississippi Valley-type (MVT) model is the most obvious explanation for mineralization. Overlying the ore-bearing sequence are organic-rich semipelites and massive calcitic marbles, which may have served as seals for hydrocarbon or reduced sulfur and acted as chemical traps for deposition of the sulfidic ore. The Mârmorilik Formation contained an interlayered sulfate-rich evaporite-carbonate sequence, a common setting for MVT deposits in the late Neoproterozoic and Phanerozoic, but unique among the few known MVT deposits in the Paleoproterozoic. This ca. 1915 Ma evaporite-carbonate platform is younger than sulfate evaporites deposited during and immediately after the ca. 2220–2060 Ma Lomagundi carbon isotope excursion and records a significant seawater sulfate level during a time interval when it was assumed that it had been too low to form extensive evaporite deposits. Therefore, MVT and clastic-dominated (CD) Zn–Pb deposits in the geological record might progressively fill the apparent gap in marine sulfate evaporites and provide unique insights into Proterozoic seawater sulfate level. Considering the sequence of tectonic events that affected the Karrat basin, the mineralization took place between Nagssugtoqidian collision (< 1860 Ma) and Rinkian metamorphism (ca. 1830 Ma).
We report here high-precision multiple sulfur and iron isotope compositions for a series of mineralized samples from Ni-Cu-(PGE) sulfide deposits in the Archean Tati greenstone belt and the Phikwe Complex of eastern Botswana. Mineralized samples from the Phoenix and Selkirk Ni-Cu-(PGE) deposits in the Tati greenstone belt display slightly positive δ 34S isotope values, ranging from 0.2 to 0.8‰ V-CDT. Δ33S values of sulfides at Phoenix and Selkirk are −0.01 to −0.08‰ V-CDT, suggesting either a dominantly mantle sulfur source or effective eradication of a crustal Δ33S anomaly through equilibration with large amounts of silicate melt. In the Selebi-Phikwe belt, a granite-gneiss terrane with abundant amphibolite lenses of either volcanic and/or intrusive nature, mineralized lower grade samples from the Phikwe, Phokoje, and Dikoloti Ni-Cu-(PGE) deposits have more variable δ 34S values ranging from −3.1 to +0.3‰ and display significant mass independent anomalies (Δ33S values ranging from −0.89 to −0.27‰), suggesting that barren sulfides associated with distal or low-temperature sea-floor hydrothermal activity contributed sulfur to these deposits. Iron isotopes of sulfides from these deposits show a relatively small range of negative Δ56Fe values (−0.29 to −0.04‰), consistent with high-temperature fractionations in magmatic systems, with the exception of one sample from the Dikoloti Ni-Cu-(PGE) deposit of the Selebi-Phikwe greenstone belt, which shows a more negative δ 56Fe value of −0.61‰, consistent with assimilation of sedimentary or hydrothermal sulfides rather than fractionations in high-temperature magmatic systems. Data from this study highlight the complexity and variability that characterize ore-forming processes in magmatic systems. We suggest that the presence of sulfur-bearing lithologic units in host rocks of mafic and ultramafic intrusions may not be essential toward the assessment of the prospectivity of a province to host orthomagmatic nickel sulfides. Geologic settings without any or little sulfur in the stratigraphy, which have been traditionally neglected in terms of their prospectivity, should thus be revisited and possibly reassessed considering the potential importance of external source of sulfur to generate ore deposits.
Archaean supracrustal rocks carry a record of mass-independently fractionated S that is interpreted to be derived from UV-induced photochemical reactions in an oxygen-deficient atmosphere. Experiments with photochemical reactions of SO2 gas have provided some insight into these processes. However, reconciling experimental results with the multiple S isotopic composition of the Archaean sedimentary record has proven difficult and represents one of the outstanding issues in understanding the Archaean surface S-cycle. We present quadruple S isotope data (32S, 33S, 34S, 36S) for pyrite from Mesoarchaean carbonaceous sediments of the Dominion Group, South Africa, deposited in an acidic volcanic lake, which help reconcile observations from the Archaean sedimentary record with the results of photochemical experiments. The data, which show low Δ33S/δ34S ratios (mostly ≪ 1) and very negative Δ36S/Δ33S ratios (−4 and lower), contrast with the composition of most Archaean sedimentary sulfides and sulfates, having Δ36S/Δ33S∼−1 (the so-called 'Archaean reference array'), but match those of modern photochemical sulfate aerosols produced in the stratosphere, following super-large volcanic eruptions, and preserved in Antarctic ice. These data are also consistent with the results of UV-irradiation experiments of SO2 gas at variable gas pressure. The S isotope composition of the Dominion Group pyrite is here interpreted to reflect the products of photolysis in a low-oxygen-level atmosphere at high SO2 pressure during large volcanic eruptions, mixed with Archaean 'background' (having a composition broadly similar to the Archaean reference array) S pools. It is inferred that high sedimentation rates in a terrestrial basin resulted in an instantaneously trapped input of atmospheric S during short-lasted depositional intervals, which faithfully represents transient photochemical signals in comparison with marine sedimentary records.
Abstract The protocratonic core of the São Francisco craton assembled during the 2.1–2.0 Ga Transamazonian orogeny. Orosirian Fe‐rich sequences that extend from the northwestern border of the São Francisco protocraton (Colomi Group) to the southeast under the Espinhaço Belt (the < 1.99 Ga Serra da Serpentina Group) record the opening of an intracratonic basin with the episodically developed ferruginous waters prior to the initiation of the Espinhaço rift at 1.8 Ga. Ferruginous conditions developed again during deposition of the Canjica Iron Formation of the < 1.7 Ga Serra de São José Group in the Espinhaço rift (contemporaneously with felsic magmatism; Conceição do Mato Dentro Rhyolite and Borrachudos Granitic Suite) and extensive sandstones of the < (1666 ±32) Ma Itapanhoacanga and < (1683 ±11) Ma São João da Chapada Formations. In the upper São João da Chapada Formation, banded hematitic phyllite also records input of Fe‐rich fluids. The young age of these iron formations with respect to the conventionally accepted 1.88 Ga age for the youngest shallow‐marine Paleoproterozoic iron formations, the apparent absence of granular facies (granular iron formations), and yet shallow‐water (above fair‐weather base) depositional environment indicate that an unusual setting developed in a large basin after the Great Oxidation Event, in the aftermath of the Transamazonian orogeny. We propose that mantle plumes led to the opening of a previously unrecognized rift system, that could have caused the magmatism, supplied hydrothermal Fe and led to the opening of the Espinhaço, Pirapora, and Paramirim rifts, later obliterated by the Araçuaí orogenic belt during the Neoproterozoic to Early Paleozoic Brasiliano orogeny. The rift system did not develop into an open continental margin but probably evolved into a broad sag basin, stretching across the São Francisco and Congo cratons.
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